The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a portable device including a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In recent years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., cellular phones with functions of portable information terminals (PDA) and/or personal computers. Since the smartphones generally are highly functional as opposed to the cellular phones, it is possible to use images taken by a camera thereof in various applications.
Generally speaking, product groups of cellular phones and smartphones are often composed of products of various specifications, from products for beginner users to products for advanced users. Among them, an imaging lens for mounting in products for advanced users requires a lens configuration of high resolution, which can be applied also in high pixel count imaging elements.
As one of methods to achieve an imaging lens of high resolution, there is a method of increasing the number of lenses that compose the imaging lens. However, an increase in the number of lenses easily causes an increase in size of the imaging lens, so that such an approach is not favorable for mounting in the above-described small-sized cameras such as cellular phones and smartphones. For this reason, the imaging lenses have been developed while restraining an increase in the number of lenses as much as possible. However, with rapid technology advancement in increasing the pixel count of an imaging element in these days, it became more interesting to develop an imaging lens that can achieve high resolution rather than a short total track length of the imaging lens. For example, according to a camera unit that became newly available, it is possible to obtain quality images equivalent to those obtained by digital still cameras by attaching a camera unit having an imaging lens and an imaging element to cellular phones, smartphones, or the like, instead of mounting it in a cellular phone, smartphone, or the like as is conventional.
A lens configuration having seven lenses is slightly disadvantageous for downsizing of an imaging lens due to the large number of lenses that compose the imaging lens. However, since such lens configuration has flexibility in designs, it has potential to achieve satisfactory correction of aberrations and downsizing of the imaging lens in a balanced manner. As such imaging lens having a seven-lens configuration, there is known, for example, the one described in Patent Reference.
Patent Reference: Japanese Patent Application Publication No. 2012-155223
The imaging lens described in Patent Reference includes a first lens having a biconvex shape, a second lens that has a biconcave shape and is joined to the first lens, a third lens that is negative and has a shape of a meniscus lens directing a convex surface thereof to an object side, a fourth lens that is positive and has a shape of a meniscus lens directing a concave surface thereof to the object side, a fifth lens that is negative and has a shape of meniscus lens directing a convex surface thereof to the object side, a sixth lens having a biconvex shape, and a seventh lens having a biconcave shape, arranged in the order from the object side.
According to the imaging lens disclosed in Patent Reference, through restraining the ratio between a focal length of a first lens group composed of lenses from the first lens to the fourth lens and a focal length of a second lens group composed of the lenses from the fifth lens to the seventh lens within certain range, it is possible to achieve downsizing of the imaging lens and satisfactory correction of aberrations.
According to the imaging lens described in Patent Reference, the size of the imaging lens is small, but correction of an image surface is insufficient. Especially distortion is relatively large, there is a limit by itself in achieving a high resolution imaging lens. According to the lens configuration described in Patent Reference, it is difficult to achieve satisfactory aberration correction while downsizing the imaging lens.
Here, such difficulty is not a problem specific to the imaging lens to be mounted in cellular phones and smartphones. Rather, it is common even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can attain both downsizing and satisfactory correction of aberrations.
Further objects and advantages of the invention will be apparent from the following description of the invention.